EVOH is a useful polymer material having good oxygen barrier properties, oil resistance, antistatic properties and mechanical strength, and is widely used for various wrapping and packaging materials such as films, sheets, containers, etc. In case where EVOH is used for various wrapping and packaging materials, not only its capability for the intended applications but also its macroscopic conditions including slight discoloration, pinholes, fish eyes, streaks and surface roughening, as well as its transparency shall be taken into consideration.
EVOH has some problems. When molded in melt, it is often discolored, and its moldings often have pin holes, fish eyes and streaks and their surfaces are often roughened. To solve the problems, various methods such as those mentioned below have heretofore been proposed.
In general, EVOH is obtained by saponifying an ethylene-vinyl acetate copolymer with a caustic alkali added thereto. However, when the resulting saponified product is directly molded in melt, it is readily pyrolyzed. As a result, its melt viscosity is greatly reduced and, in addition, it is noticeably discolored. Therefore, directly using the saponified product is impossible. To solve the problem, many methods have been proposed. For example, EVOH could be improved in some degree by fully washing it with water or by dipping it in an acid or an acid solution, as so described in Japanese Patent Publication No. 37664/1971, Japanese Patent Laid-Open Nos. 25048/1973, 88544/1976, 88545/1976, Japanese Patent Publication No. 19242/1980, etc.
Some types of metal salts are extremely effective for improving the thermal stability of EVOH, and adding a metal salt to EVOH to improve the melt moldability of EVOH is disclosed, for example, in Japanese Patent Laid-Open Nos. 954/1977, 955/1977, 41204/1981, etc.
Adding olefins, vinylsilane-type compound-copolymerized polyolefins and the like to EVOH to remove streaks from EVOH moldings owing to their elastic effect is disclosed in Japanese Patent Laid-Open No. 197138/1991.
In Japanese Patent Laid-Open No. 66262/1989 (USP 5,118,743), described is an EVOH resin composition that contains from 5 to 500 ppm, in terms of the metal, of a salt of a metal of Group 2 of the Periodic Table, from 20 to 2000 ppm of an acidic substance having a boiling point of not lower than 180.degree. C., and from 100 to 2000 ppm of an acidic substance having a boiling point of not higher than 120.degree. C.
In that patent publication, boric acid is referred to for the acidic substance having a boiling point of not lower than 180.degree. C., and acetic acid is for the acidic substance having a boiling point of not higher than 120.degree. C. However, these are among a large number of compounds exemplified in the publication. The content of the salt of a metal of Group 2 of the Periodic Table in the resin composition shown in the Examples is 100 ppm or more, in terms of the metal, and it is far larger than the content thereof that may be preferred for the composition of the present invention.
When the EVOH composition processed according to the proposed method is molded in an ordinary melt extrusion process (at an extrusion temperature higher by about 20.degree. C. than the melting point of EVOH), its moldability could be improved in some degree. However, when it is molded under severe conditions, for example, at a temperature higher by 30.degree. C. or more, especially by 50.degree. C. or more, than the melting point of EVOH, it is noticeably degraded in the extrusion molding machine. As a result, the molded films will have many fish eyes and streaks, and their appearances are bad. After all, practicable moldings could not be obtained.
Co-extrusion of EVOH with other thermoplastic resins to produce multilayer structures has been popularized. Multilayer structures require interlayer adhesiveness between the EVOH layer and the adjacent resin layer, for which, therefore, needed is EVOH capable of ensuring the interlayer adhesiveness and having high-level melt stability, especially improved, long-lasting melt stability even at high temperatures. From this viewpoint, the properties of various types of EVOHs mentioned above are unsatisfactory.
In case where EVOH is extruded and molded at a high speed in order to ensure good productivity, it is often advantageous to lower its melt viscosity. This is for increasing the resin output at a predetermined level of energy applied. In such high-speed extrusion, the resin will have to be melted and molded at high temperatures, at which, however, the resin melt will be greatly degraded.
In particular, in case where a multilayer melt containing EVOH is coated on a substrate such as paper or the like, the EVOH layer to be formed must be thin and uniform so as to reduce the production costs and to ensure the gas barrier properties of the layered structures. In that case, the resin will be much degraded and its moldings will have many defects.
In Japanese Patent Laid-Open No. 192564/1984, disclosed is a laminate structure comprising a layer of boric acid-containing EVOH and a layer of a carboxylic acid-modified polyolefin. In this, boric acid is added to EVOH so as to improve the interlayer adhesiveness of the laminate structure. Nothing is referred to in the patent publication, relating to the melt stability of EVOH. In addition, nothing is referred to therein, relating to any other minor components except boric acid that may be added to EVOH.
In Japanese Patent Laid-Open No. 234645/1992, disclosed is co-extrusion coating of a multilayer melt that comprises EVOH and an acid-modified polyolefin, on a paper substrate. In this, however, nothing is referred to, relating to the preferred condition of EVOH.
In the background noted above, the object of the present invention is to provide multilayer structures which are produced through co-extrusion of a multilayer melt that comprises a layer of an ethylene-vinyl alcohol copolymer (hereinafter often referred to as EVOH) resin composition and a layer of a carboxylic acid-modified polyolefin resin adjacent thereto and which have the advantages of improved thermal stability (long-run workability) in their production, especially in producing them through co-extrusion at high temperatures, improved co-extrusion molding stability especially in high-speed molding lines, and improved interlayer adhesiveness, and also to provide a process for producing them, and resin compositions suitable to them.